The Guardian quotes Graziano Cecchetti, Senior Associate at Giambrone

Amanda Knox and Raffaele Sollecito face new legal challenge after decision by the Court of Cassation

The highest court in Italy has ordered a fresh trial over the death of the British student Meredith Kercher, two years after Amanda Knox and her former boyfriend Raffaele Sollecito were acquitted of the killing in a case that gripped Italy, Britain and the US.

The decision by the court of cassation, which raises the prospect of an extradition tussle between Italy and the US, was welcomed by Kercher's family in Britain but met with incredulity and anger in Seattle, where Knox, now 25, lives.

"There are a lot of unanswered questions still," Kercher's older sister, Stephanie, said. "We are very hopeful that it going back to court will help find those answers and find out the truth of what happened that night."

Kercher, 21, from Coulsdon, Surrey, was found dead in the flat she shared with Knox in the university town of Perugia in November 2007.

Rudy Guede, from Ivory Coast, was found guilty of sexual assault and murder in October 2008. But, driven by the argument that he did not act alone, prosecutors also pursued their case against Knox and Sollecito, who were eventually sentenced, respectively, to 26 and 25 years in jail.

It was after they had spent four years behind bars that they were eventually freed, in 2011, after a court found, among other things, that DNA evidence used to convict them had been unreliable.

The new proceedings will not substitute the original trial but will be a fresh appeal of the pair's 2009 convictions.

Knox, who was 20 when she was arrested by police in 2007, condemned the court of cassation's decision to allow a fresh appeal on Tuesday, describing the prosecution's case against her as "unfounded and unfair".

Knox said: "It was painful to receive the news that the Italian supreme court decided to send my case back for revision when the prosecution's theory of my involvement in Meredith's murder has been repeatedly revealed to be completely unfounded and unfair.

She added: "No matter what happens, my family and I will face this continuing legal battle as we always have, confident in the truth and with our heads held high in the face of wrongful accusations and unreasonable adversity."

Giulia Bongiorno, Sollecito's defence lawyer, said that it was the 29-year-old's birthday and she would have liked to have had better news for him. "Unfortunately, we have to continue the battle," she said.

It was not within the court of cassation's remit to base its decision on the presumed guilt or innocence of Knox or Sollecito; its ruling hinged solely on whether or not the appeal trial against the pair's 2009 convictions had been properly conducted.

Bongiorno said: "Quashing an acquittal does not imply a belief that the accused are guilty but indicates a desire by the judges of the court of cassation to request further analysis." The new trial, which will take place in the appeal court in Florence, is unlikely to begin before early next year.

If Knox, who has enrolled at the University of Washington and has recently written her memoirs in a deal reportedly worth $4m (£2.6m), were to have her appeal turned down and her previous conviction upheld on appeal, Italy could request her extradition. It would then be up to the US to decide whether to agree to the move, something some observers say is unlikely. Her presence would not be required at the fresh appeal trial in Florence.

Soon after phoning his client with the decision, Carlo Dalla Vedova, Knox's lawyer, said on the steps of the court: "She thought that the nightmare was over. [But] she's ready to fight."

The specific reasons for the latest ruling will remain unclear until the court releases the judges' reasoning. But, on Monday, prosecutors argued that the Perugia court that acquitted Knox and Sollecito had "lost its bearings" in the case and had erred in numerous ways, including demanding insufficient forensic evidence tests.

Luigi Riello, the state prosecutor, criticised what he said was a mass of illogical reasoning and legal violations. "I believe all the elements are there to make sure the final curtain does not drop on this shocking and dreadful crime."

In its dramatic acquittals of the former lovers in 2011, the Perugia court found fault with virtually every aspect of the original investigation into Kercher's murder.

The alleged mishandling of the first investigation was returned to by Bongiorno on Monday, who, as Sollecito's father looked on, reminded the judges that "an infinite series of errors" had been made by forensics. A bra clasp found to have a trace of Sollecito's DNA on it, which formed a central plank of the prosecution's case, had been bagged no fewer than 47 days after the inquiry began, she noted.

But it was with the arguments coming from the other side of the court that the judges eventually sided. Francesco Maresca, lawyer for the Kerchers, lambasted the case behind the acquittals as "absolutely superficial" and said evidence had been ignored. He firmly believed, he added, that "there were more people in that room than Rudy Guede".

In her statement, Knox said claims against her should now be examined by "an objective investigation and a capable prosecution". "The prosecution responsible for the many discrepancies in their work must be made to answer for them, for Raffaele's sake, my sake, and most especially for the sake of Meredith's family," she said. "Our hearts go out to them."

Meanwhile, Stephanie, Meredith's sister, said the family had to "make sure we have done all we can for [Meredith]".

"We still have a long journey ahead and we are very grateful for the support of the public and in Italy," she said. "We just want justice for Mez."

What next for Knox?

Amanda Knox will, in all likelihood, be staying put for the moment. She will not be required to attend the fresh appeal trial in Florence. Even if her conviction is upheld, her lawyers could then take her case back to the court of cassation for a final appeal while she remained in the United States.

If, however, her conviction were not overturned at that point – and that is a very big "if" at this stage – the Italian government could then choose to request her extradition from her home country. So will Knox, whose year abroad in Umbria went so tragically wrong, end up back in Italy? Unlikely, say most observers.

"Having followed this case from the beginning I would say America will not allow extradition … or at least I would say it's not going to be easy for Italy to obtain," said GrazianoCecchetti, Senior Associate at Giambrone, a London-based Italian lawyer.

The reasons for this are not only that the US would be unlikely to grant the request because of, for instance, previous criticism of the trial, but also that Italy may well not make the request in the first place, knowing how diplomatically sensitive the high-profile case would be.

Some say that if Knox did find herself in this position, a deal may well be done to allow her to serve her time in the US. But she is still a long way off having to confront that